"The regime has lost the last oilfield in Syria," said the Britain-based Syrian Observatory for Human Rights, which tracks violence through a network of sources on the ground.
The Jazal field was now shut down and clashes were ongoing east of Homs, with casualties reported on both sides, the Observatory said, without giving dates or more details.
Syria's army said it had repulsed an attack in the area close to the Jazal field. It said it had killed 25 fighters, including non-Syrian jihadists. The army claims much of the country's battered energy infrastructure remains under its control.
Jazal is a medium-sized field that lies to the north west of the rebel-held ancient city of Palmyra, close to a region that holds Syria's main natural gas fields and multi-million-dollar extraction facilities.
The Observatory also said U.S.-led coalition bombing raids in areas in the militant's de facto capital of Raqqa had killed at least 16 militants, including five foreign jihadists, according to the report.
